1. Field of the Invention
The invention relates to a network device, and more particularly to a network device with a hybrid-mode transmitter.
2. Description of the Related Art
The network application becomes more and more popular and becomes an indispensable portion in the daily life owing to the progress of the technology. In view of the increasing demands on the network bandwidth, such as the application of the on-line multimedia, the transmission rate of the network device has to be risen from the conventional 10/100 Mbs to 1 Gbs or more.
Take the network device with the rate of 1 Gbs as an example. The 1 Gbs network device has at least one port, and each port has four channels. Each one channel is coupled to a twisted pair, in order to transfer a differential transmission signal. The network device uses the four channels to communicate with another remote network device, wherein the transmitting and receiving functions are simultaneously performed in each of the channels.
A typical 1 Gbs network device has the capability downward compatible with 10/100 Mbs. When the network device is operating at the rate of 10/100 Mbs, one port only needs two channels, one of which is for transmission, and the other of which is receiving. If the network device is operating at 10 Mbs, the peak-to-peak value of the transmitted voltage is 5V. If the network device is operating at 100 Mbs/1 Gbs, the peak-to-peak value of the transmitted voltage is 2V.
In general, the transmitter in the channel is the most power-consumptive component in the network device, so the manufacturer is committed to reduce the power consumption of the transmitter in order to save the power.
The transmitters in the channels may be classified into a current mode and a voltage mode. FIG. 1A is a schematic illustration showing a conventional network device having current-mode transmitters. Herein, a network device 100 with one port will be described. The network device 100 includes a physical layer (PHY) control chip 110, a plurality of matching resistors Ri and transformers T. The physical layer control chip 110 has one port, which includes four channels 112. Each channel 112 includes a control unit and a transceiver 114, as shown in FIG. 1A. Each transmission channel has two input/output pins P to be electrically connected to a primary side of the transformers T and the matching resistors Ri. The transformer T filters out the DC component of the input or output signal. The central tap of the coil at the primary side of the transformer T is coupled to the DC power VDD, while the secondary side thereof is coupled to a twisted pair of the Ethernet. The impedance value ZL of the wire is 100 Ω, so the resistance value of the matching resistor Ri is also 100 Ω. Each channel in the current mode network device 100 has two input/output pins P, so one port includes eight input/output pins P.
FIG. 1B is a schematic illustration showing a transmitter 120 for transferring the differential transmission signal in the transceiver 114 of FIG. 1A. The transmitter 120 is an open-drain current driver, which includes a first input terminal SI1 a second input terminal SI2, transistors N1 and N2, and a current source I coupled to the transistors N1 and N2. The first input terminal SI1 and the second input terminal SI2 are for receiving input signal S. The gate electrodes of the transistors N1 and N2 are electrically connected to the first input terminal SI1 and the second input terminal SI2, respectively. The drain electrodes of the transistors N1 and N2 are electrically connected to the two input/output pins P, respectively. The source electrodes of the transistors N1 and N2 are electrically connected to the current source I. The input signal S are digital signals controlling the ON and OFF of the transistors N1 and N2, respectively. The transmitter 120 amplifies the input signal S and outputs differential transmission signals Tx+ and Tx−. Because the matching resistor Ri connected to the output terminals of the transmitter 120 in parallel is 100 Ω and the matching resistor Ri is connected to the impedance ZL of the twisted pair in parallel, the equivalent impedance of both of them are 50 Ω. Consequently, the current source I has to provide the 40 mA current so as to provide the peak-to-peak output voltage of 2V during the 100 Mbs/1 Gbs operation.
FIG. 2A is a schematic illustration showing a conventional network device having voltage-mode transmitters. The network device 200 having one port will be illustrated as an example. The network device 200 includes a physical layer control chip 210, a plurality of matching resistors Rv and transformers T. The physical layer control chip 210 has one port, which includes four channels 212. Each channel 212 includes a control unit and a transceiver 214. Each channel 212 includes four input/output pins P to be electrically connected to the corresponding matching resistors Rv at the primary sides of the transformers T. The transformer T is for filtering out the DC component of the received or output signal. The secondary sides of the transformers T are coupled to the twisted pairs of the Ethernet. The value of the impedance ZL of the wire is 100 Ω, so the resistance value of the cascaded matching resistor Rv in each channel is 500. One port of the voltage mode network device 200 needs 16 input/output pins P.
FIG. 2B is a schematic illustration showing a transmitter 222 in the transceiver 214 of FIG. 2A. The transmitter 222 includes a first input terminal SV1, a second input terminal SV2, a differential operational amplifier OP, and feedback resistors Rf1 and Rf2. The first input terminal SV1 and the second input terminal SV2 receive the input signal S. The differential operational amplifier OP has a noninverting input terminal and an inverting input terminal respectively electrically connected to the first input terminal SV1 and the second input terminal SV2. The differential operational amplifier OP further has a first output terminal and a second output terminal. Unlike the above-mentioned current-mode transmitter, the input signal S respectively inputted to the first input terminal SV1 and the second input terminal SV2 in the voltage-mode transmitter are analog current signals. The differential operational amplifier OP amplifies the input signal S and then generates differential output signals Vo1 and Vo2. The output signal Vo1 of the first output terminal is inverse to the output signal Vo2 of the second output terminal. The differential output signals Vo1 and Vo2 of the differential operational amplifier OP are fed back to the first input terminal SV1 and the second input terminal SV2 through the feedback resistors Rf1 and Rf2, respectively. Two matching resistors Rv are electrically connected to the output terminals of the differential operational amplifier OP, and the output terminals of the differential operational amplifier OP have low impedances to match with the impedance ZL of the twisted pair. Because the matching resistors Rv and the impedance ZL form a voltage-dividing circuit, if the peak-to-peak value of the output signal of the amplifier OP is (Vo1-Vo2), then peak value of the differential transmission signal (Tx+)-(Tx−) is only one half of (Vo1-Vo2), i.e., (½)*(Vo1-Vo2). If the peak-to- peak value between the differential transmission signals Tx+ and Tx− needs to be 2V, as specified by the specification of the 100 Mbs/1 Gbs network device, then the peak-to-peak value outputted from the differential operational amplifier OP has to be 2×2=4V. If the peak-to-peak value between the differential transmission signals Tx+ and Tx− has to be 5V, as specified by the specification of the 10 Mbs network device, then the peak-to-peak value outputted from the differential operational amplifier OP has to be 5V×2=10V. Hence, the voltage required by the voltage-mode transmitter is very high.
In addition, when the network device is operating at 1 Gbs, each twisted pair receives differential receiving signals from the ethernet and transmits differential transmission signals, Tx+ and Tx−, simultaneously. Therefore, the receiver 224 receives a coupled differential signals, Rx+ and Rx−, which are the coupling of the differential receiving signals and the differential transmission signals Tx+ and Tx−. The differential receiving signal is not coupled to the output signals Vo1 and Vo2 of the amplifier OP, and the receiver 224 obtains the differential receiving signals by echo cancellation, that is, by subtracting the differential transmission signals, Tx+ and Tx−, according to the signals Vo1 and Vo2, from the coupled differential signals, Rx+ and Rx−.
However, each voltage mode transceiver 214 requires two more input/output pins than each current mode transceiver 114 because the input terminals of the receiver 224 for the coupled differential signal is outside of the physical layer control chip 210 and needs to externally coupled precise resistors to obtain exact divided voltage.
The drawback of the current-mode transmitter is the great power consumption. As for the network device operating at 1 Gbs, one channel needs the current of 40 mA, and one port, which has four channels, thus needs the current of 160 mA. If the switch has four ports, the current of 640 mA is consumed. In addition, the consumed current of the current-mode transmitter is independent of the output signals because such current has to be supplied no matter the output signal is 0 or 1.
The drawbacks of the voltage-mode transmitter are that its voltage swing is larger and input/output pins are more than the current-mode transmitter. With the advance of the IC (integrated circuit) manufacturing process, the supplied voltage to the IC is getting smaller (e.g., 1.8V) for saving power. Consequently, the too-large voltage swing causes the difficulty of implementing the low-voltage integrated circuit.